Rendering to multichannel .exr has empty rgb layer
The OpenEXR file format stores pixel data in 2 ways (Scan Lines or Tiles), and read/writes data in 2 modes (Arbitrary Channels or RGBA Only). 








Excerpt from http://www.openexr.com/ReadingAndWritingImageFiles.pdf



In our case, Blender only stores data via the Scan Line method, so our main focus in our the read/write interface in use.

When the OpenEXR format is chosen in Blender, the output class used is the OutputFile class, and the EXR format used is the RGBA Only format which renders out the single beauty pass with no channel information. This means that it only contains a single RGBA frame buffer.









Excerpt from http://www.openexr.com/ReadingAndWritingImageFiles.pdf:


When the OpenEXR Multilayer format is chosen in Blender, the output class used is the RgbaOutputFile class, and the EXR format used is the Arbitrary Channel mode which organizes all image data into Layers and Passes/Channels. There is no RGBA buffer that does not exist inside a Layer and Pass/Channel.









Excerpt from http://www.openexr.com/ReadingAndWritingImageFiles.pdf:



If we have an arbitrary Layer named "Foreground", then Blender automatically calls the beauty pass/channel "Combined" (we don't have control over the channel name, unfortunately). The image data would be defined as "Foreground.Combined.R", "Foreground.Combined.G", etc.

If Nuke is ingesting a MultiChannel EXR file, it should not be trying to read a master RGBA beauty pass because none exists. Image data only exists at the Layer.Channel level. 

It probably would be more helpful if Blender could name a Layer as "rgb" (which it can) and the beauty pass "rgba" instead of "Combined" (which it can't).